Understanding union query

Mastering UNION and UNION ALL in SQL: Combining Result Sets

Explore the fundamental differences and practical applications of UNION and UNION ALL operators in SQL to efficiently combine data from multiple queries.

In SQL, the UNION and UNION ALL operators are powerful tools for combining the result sets of two or more SELECT statements into a single result set. While both serve the purpose of merging data, they have a crucial distinction related to duplicate rows. Understanding when to use each operator is key to writing efficient and accurate SQL queries.

The Basics of UNION and UNION ALL

Both UNION and UNION ALL require that the SELECT statements have the same number of columns, and the corresponding columns must have compatible data types. The column names in the final result set are determined by the column names from the first SELECT statement. The primary difference lies in how they handle duplicate rows.

flowchart TD
    A[Query 1] --> B{Combine Results}
    C[Query 2] --> B
    B --> D{Remove Duplicates?}
    D -- Yes --> E[UNION]
    D -- No --> F[UNION ALL]
    E --> G[Final Result Set (Unique)]
    F --> H[Final Result Set (All Rows)]

Decision flow for choosing between UNION and UNION ALL

UNION: Combining and Deduplicating

The UNION operator combines the result sets of two or more SELECT statements and then removes any duplicate rows from the final result. This means if a row exists in both result sets, or multiple times within a single result set, it will appear only once in the UNION output. This deduplication process can be resource-intensive, especially with large datasets, as the database engine needs to sort and compare all rows to identify and remove duplicates.

SELECT CustomerID, FirstName, LastName
FROM Customers
WHERE Country = 'USA'

UNION

SELECT EmployeeID, FirstName, LastName
FROM Employees
WHERE Department = 'Sales';

Example of UNION combining customer and employee names from USA/Sales, removing duplicates.

UNION ALL: Combining All Rows

In contrast, the UNION ALL operator combines the result sets of two or more SELECT statements without removing duplicate rows. Every row from each SELECT statement is included in the final result set, even if it is an exact duplicate of another row. This makes UNION ALL generally faster and more efficient than UNION because it avoids the overhead of sorting and deduplicating the data. You should use UNION ALL when you know there are no duplicates, or when you explicitly want to retain all duplicate rows.

SELECT ProductID, ProductName, Price
FROM Products_WarehouseA

UNION ALL

SELECT ProductID, ProductName, Price
FROM Products_WarehouseB;

Example of UNION ALL combining product lists from two warehouses, including potential duplicates.

Key Considerations and Best Practices

When working with UNION and UNION ALL, keep the following points in mind to ensure your queries are correct and performant:

1. Column Count and Data Types

Ensure that all SELECT statements involved in the UNION or UNION ALL operation have the same number of columns, and that the data types of corresponding columns are compatible. Implicit conversions might occur, but it's best practice to explicitly cast if types differ significantly.

2. Column Naming

The column names in the final result set will be derived from the first SELECT statement. If you need specific column names, use aliases in the first SELECT statement.

3. ORDER BY Clause

An ORDER BY clause can only be used at the very end of the entire UNION or UNION ALL query, not within individual SELECT statements (unless used within a subquery). It will sort the combined result set.

4. Performance Implications

UNION involves an implicit DISTINCT operation, which requires sorting the entire combined dataset. This can be very slow for large tables. UNION ALL simply appends the results, making it much faster. Use UNION only when deduplication is strictly necessary.